This invention is in the field of chemical processes; more specifically, this invention is an improvement in a known process for making carbamates, organic compounds of the general formula: ##STR1##
Carbamates are well known compounds having established utilities as drugs, insecticides, and as polymer precursors, for example. Carbamate insecticides are listed in various indexes; for example, "Chemical Week Pesticides Register,"McGraw-Hill Book Co., New York, N.Y., 1977 and "Pesticide Index," The Entomological Society of America, College Park, Md., 1976. Such insecticides include, for example, butacarb, carbofuran, carbaryl, terbucarb, ethiofencarb, bufencarb, isoprocarb, aminocarb, bendiocarb, dioxacarb, formetanate, methiocarb, promecarb, propoxur, dicresyl, and MPMC, which are common names for carbamates of the aforesaid formula wherein R.sup.1 is methyl, R.sup.2 is 3,5-di-tert-butylphenyl, 2,3-dihydro-2,2-dimethyl-7-benzofuranyl, 1-naphthyl, 2,6-di-tert-butyl-4-methylphenyl, 2-ethylthiomethylphenyl, 3-(1-methylbutyl)phenyl and 3-(1-ethylpropyl)phenyl (i.e., a mixture), 2-isopropylphenyl, 4-dimethylamino-3-methylphenyl, 2,2-dimethyl-1,3-benzodioxol-4-yl, 2-(1,3-dioxolan-2-yl)phenyl, 3-dimethylaminomethyleneiminophenyl, 3,5-dimethyl-4-(methylthio)phenyl, 3-isopropyl-5-methylphenyl, 2-(1-methylethoxy)phenyl, 3-methylphenyl, and 3,4-dimethylphenyl, respectively, and R.sup.3 is hydrogen.
The aforesaid carbamates are prepared in several ways. For example, they are prepared by condensing an alcohol or phenol, R.sup.2 OH, with a previously prepared isocyanate, R.sup.1 NCO, or carbamoyl chloride, R.sup.1 NHCOCl, or with phosgene first and subsequent reaction of the chloroformate thus produced with an amine, R.sup.1 NH.sub.2.
U.S. Pat. No. 4,086,246 discloses a process for making carbamates in one step from commercially available materials, wherein the phenol, R.sup.2 OH, phosgene, and an amine, R.sup.1 NH.sub.2, are all reacted in a common water-immiscible organic solvent in the same reaction vessel at the same time in the presence of an acid binding agent to yield the carbamate. The acid binding agent, generally a tertiary amine, is a scavenger for the by-product hydrogen chloride.
The scavenged hydrogen chloride salt is a troublesome by-product, difficult to remove from the reaction mixture and costly to dispose of without contaminating the environment. Furthermore, in order to avoid the formation of 1,3-dimethylurea, the prior art has employed large amounts of phosgene, creating further separation and disposal problems as well as higher costs. Thus, the aforesaid requirements in the one-step process of the prior art seriously limit the commercial utility of that process. The economic advantage of a one-step process over two steps via an isocyanate, carbamoyl chloride, or chloroformate intermediate will be evident.